On the Underwater Noise from Fishing Boats and Netting Gear in Course of Fishing Operation

摘要

Resently, investigation has been made into the relationship between underwater noises and fish catches and into the fishing method using sounds. As one of the fundamental problems on these subjects, it is necessary to clarify the acoustical features of the noises in the spaces where fishes are distributed, for example, the frequency distribution and the sound pressure of the underwater noises which are made by fishing boats and gear. Therefore the author has measured by the aid of the underwater tape-recorder the underwater noises in course of the fishing operation with the one-boat type trawl net, the bull trawl net, and the purse seine. The analytical results of the measurement are as follows: 1) The underwater noises measured in fishing with the three types of fishing gear were different from one another. The noise from the one-boat type trawl net was pulsating, whereas that from the bull trawl net was almost continuous. The noise from the purse seine was also continuous being similar to that from the bull trawl net, though its frequency band was narrower than that for both types of trawl nets. 2) The frequency distribution of the noise was DC to 5000 Hz in the case of both trawl nets, and DC to 3000Hz in the case of the purse seine. The most predominant frequencies in these ranges were 500Hz to 800Hz in the case of the one-boat type trawl net, and 400Hz to 600Hz, somewhat lower though not so remarkable, in the case of the bull trawl net. In the case of the purse seine, on the other hand, any especially predominant frequency was not seen, but its peculiar noise of 5400Hz to 6000Hz caused by the friction of the purse ring against the purse wire was perceived in bracing the sinker side. 3) The acoustic comparison of the noises in fishing among the three type of netting gear showed that the engine noise was inaudible in the case of the one-boat type trawl net because of other noise, but was clearly perceived in the cases of the bull trawl net and the purse seine. 4) concerning the fish catch with the one-boat type trawl net in the East China Sea, the weight compositions of several useful species such as the drums (sciaenid fishes) were investigated. From the mean weight the resonance frequency of the air bladder was calculated and was revealed to almost correspond with the predominant frequency of the underwater noises given forth by the netting gear. In consequence, it is well considered that these underwater noise have significant influences on the perception and behavior of fish.